The invention concerns an installation intended to ensure the storage over the very long term of calorific products likely to release large quantities of heat which may decrease in time.
The term xe2x80x9cstoragexe2x80x9d designates the reversible storage of packaged products, accompanied by evacuation of the heat released by these products. By the adjective xe2x80x9creversiblexe2x80x9d is meant that the stored products may be taken out from storage.
The expression xe2x80x9cvery long-termxe2x80x9d means at least fifty years and, preferably, several periods of fifty years.
One privileged application of the installation of the invention concerns the storage of nuclear waste having very high long-term activity, such as irradiated fuel in nuclear reactors.
The storage of hazardous calorific products such as nuclear waste is a major problem for which a certain number of solutions have already been put forward.
Among such solutions, reference will only be made to those which ensure passive cooling of the products without the supply of any outside energy. This passive form appears essential to obtain the required reliability throughout the very long period of storage under consideration.
According to one first known storage technique, the products are packaged in containers which are placed in cavities made in the ground, said cavities being delimited by concrete walls. An air-filled space is provided between each container and the cavity walls. Heat evacuation is obtained solely by circulation of air under natural convection.
One notable disadvantage of said type of installation is that cooling is achieved via a primary circuit, in direct contact with the container walls. This type of arrangement is dispersive in the event of an incident and therefore dangerous for the environment. In addition, it only allows very limited evacuation of the heat flow.
According to another known storage technique, the general arrangement is similar to the previous one, but cooling is ensured by secondary cooling circuits through which a fluid passes, water in particular or air under natural convection. These circuits are fully embedded in the concrete walls which delimit the cavities housing the containers.
Such installations have a certain number of disadvantages.
Firstly, since cooling is achieved only inside the concrete walls themselves, the surfaces of these walls delimiting the cavities are heated directly by the stored products. The consequence is weakening of the concrete at least on the surface. Also, the temperature of the containers remains very high, leading to rapid ageing of their welds. Finally, with such storage installations it is not possible to control the outside temperature and therefore the inside temperature of the containers and this may, for example, lead to destruction of the cladding of the irradiated fuel.
A third known storage technique sets itself apart from the previous technique chiefly through the fact that the secondary cooling circuits cross through the walls delimiting the cavities and are partly located in the space surrounding the containers.
In this case, almost the same disadvantages are found as with the previous known technique. Also, since the cooling circuit passes locally through the surfaces of the concrete walls delimiting the cavities, these surfaces are subjected to non-homogeneous heat stresses which lead to accelerated ageing of the concrete.
With a fourth known storage technique, the space provided between each container and the cavity in which it is housed is filled with water and the cooling circuit is fully located in this space.
This known solution is characterized by corrosion problems due to the fact that the containers are immersed in water. Also, any leak from the cooling circuit entails a contamination risk if the stored products are nuclear waste. Further, the maintenance of this type of storage device is particularly heavy.
From document DD-A-223 562 an installation is known for the storage of irradiated nuclear fuel in which cylindrical containers containing the products are placed one on top of the other in wells delimited by concrete walls. The wall of each well is lined on the inside with a metal tube which projects above the well as far as a heat dissipater, with vanes or similar, able to transmit the heat it receives to the surrounding atmosphere. A plug is placed at the top of the well inside the metal tube above the containers.
The efficacy of said device is relatively limited and does not prevent major heating of the containers and well walls. Also, a substantial heat gradient exists between the containers placed at the bottom of the well and the containers nearer to the surface. Consequently, surface weakening of the concrete and accelerated ageing of the container welds and dissipater tube (which is not interchangeable) are practically unavoidable.
Also, document U.S. Pat. No. 4,040,480 describes a storage installation for radioactive products in which the products are packaged in cylindrical containers and placed in a ring-shaped cavity delimited between the concrete wall of a well having a circular cross-section and a closed vertical tube, forming a coolant duct, positioned in the well axis. At its top part, positioned above a plug sealing the well, the vertical tube carries cooling vanes in contact with the air.
The heat diffused by the stored products propagates both towards the well wall and towards the tube forming the coolant duct. Relatively rapid damage to the concrete surface is therefore predictable. Also, no provision is made in the event of failure of the coolant duct.
As a general rule, the installations known to date are designed for a maximum lifetime of approximately fifty years, whereas the need exists in the nuclear industry for storage over several fifty-year periods, typically up to 300 years.
Document JP-A-05 273393 suggests packaging spent fuel assemblies separately in casings and placing each of the casings in a closed container hung from a slab of a building. The lower part of each container is housed in an individual well and the top part is positioned in a common corridor swept by a stream of coolant air.
Finally, document FR-A-2 160 concerns a transport tower for radioactive products surrounded by a jacket fitted with cooling vanes, the jacket being assembled such that it can be dismounted.
The subject of the present invention is precisely a storage installation for calorific products, such as nuclear waste, which does not have the disadvantages of installations of the prior art. In other words, the subject of the invention is a passive storage installation able to evacuate a great quantity of heat over a very long period, while offering very high reliability and sturdiness, in particular by only subjecting the materials to demands that are compatible with a very long lifetime.
In accordance with the invention, this result is obtained by means of a very long-term storage installation for calorific products, comprising at least one sealed cavity, at least one confinement container for said products, able to be housed in the cavity, and means forming a thermosiphon able to dissipate the heat released by said products above the cavity, characterized in that the thermosiphon-forming means are partly integrated in a jacket in direct contact with the container which it surrounds.
The use of means forming a thermosiphon integrated into a jacket closely surrounding the container makes it possible to provide efficient evacuation of the heat released by the products contained in the container, without however risking any dispersion of contamination in the event of an accident. Also, the jacket forms a heat shield between the container and the wall of the cavity. The latter, generally made in concrete if the stored products are nuclear waste, is therefore cooled efficiently and in homogeneous manner in the same way as the actual container. Accelerated ageing of the concrete, container welds and container contents is therefore avoided. In addition, it is possible to have knowledge of and efficiently adjust the surface temperature of the container and the temperature of the wall of the well or trench. This also makes it possible to pilot the conditions of storage in accordance with usual hypotheses (not generally heeded in existing installations) according to which the temperature of the concrete surface is known and fixed. Such installation also has the advantage of allowing the cold source, positioned above the cavity, to be adapted to changes over time in the heat released by the stored products.
In one preferred embodiment of the invention, the jacket can be dismounted. Also, the cavity is advantageously sealed by a removable plug above the container. With this arrangement it is possible, if necessary, to replace the jacket integrating the thermosiphon or to remove the container should any problems arise.
In this case, the jacket is advantageously open and made in a flexible, elastic material such as metal so that it can occupy a natural state in which it is spaced away from the container. In this natural state the jacket can be easily mounted and dismounted. In this case, releasable clamp means are provided, to apply the jacket tightly around the container at the time of placing in storage.
Preferably, the jacket is then in the shape of a cylinder open along a generating line and the releasable clamp means are positioned between the edges vis-a-vis this generating line.
In order to avoid excessive heating of the cavity walls, a space generally filled with air is advantageously provided inside the cavity around the container fitted with its jacket, and the air may or may not be circulated by natural convection.
In the preferred embodiment of the invention, the jacket comprises a plurality of outer tubes filled with coolant fluid whose lower and upper ends respectively lead to a lower ring collector and an upper ring collector.
In this case, cooling vanes are preferably formed on at least some of the outer tubes, such as to increase heat exchange with the air contained in the cavity.
In this preferred embodiment of the invention, the outer tubes may be welded to the jacket.
As a variant, the jacket may also comprise a plurality of segments, fixed end to end by assembly means such as welds or rivets. Each of the outer tubes is then made in one piece with one of these segments.
In order to ensure cooling of the fluid (generally water) contained in the thermosiphon-forming means, the latter also comprise heat exchange means placed above the cavity and forming a cold source.
If the jacket can be dismounted, the heat exchange means are connected to the jacket by connection means which can be disconnected.
Advantageously, the heat exchange means are adapted to variations in the flow of heat to be dissipated.
In the preferred embodiment of the invention, the thermosiphon-forming means form a coolant duct.
Advantageously, the installation of the invention is applied to the storage of nuclear waste. In this case, the cavity is delimited by concrete walls.